1. Field of the Invention
The present invention relates to an automatic transmission and more particularly to a continuously variable automatic transmission whose overall width is shortened so as to be accommodated in a limited space of an engine room.
2. Prior Arts
In general, a continuously variable automatic transmission used for a motor vehicle comprises a continuously variable transmission which is a main transmission mechanism for controlling vehicle speeds during normal operation, a starting apparatus for controlling power transmission of an engine and transmission during the vehicle starting period, and a forward and reverse changeover apparatus for changing over from forward to reverse running.
The starting apparatus described herein has a function of a clutch for transmitting power from the engine to the transmission smoothly when the vehicle starts to move from a standstill condition.
For example, Japanese Patent Application Laid-open No. Toku-Kai-Shou 61-105333 discloses a continuously variable automatic transmission using a torque converter as a starting apparatus, as shown in FIG. 2.
When the engine starts, the driving force of the engine 10 is transmitted to a pump impeller 2c incorporated in a torque converter 2b through a torque converter case 2a provided in the starting apparatus 2. Then, the pump impeller 2c drives a turbine runner 2d via working fluid and at the same time drives an oil pump 2e.
Further, there is provided with a forward and reverse changeover apparatus 4 between the starting apparatus 2 and a continuously variable transmission 3 and an output shaft 2f connected with the turbine runner 2d is interconnected with a forward clutch 4a.
During forward running, the forward clutch 4a is engaged to transmit the driving force to a primary pulley 3a through the forward clutch 4a.
On the other hand, during reverse running, the forward clutch 4a is released and at the same time a reverse brake 4b is engaged to fix the rotation of a ring gear 4d of the planetary gear 4c. As a result, a sun gear connected with the primary pulley 3a of the continuously variable transmission 3 is rotated reversely at reduced speeds through a planetary pinion connected with a planetary carrier 4f which is connected integrally with a clutch drum 4e of the forward clutch 4a to rotate the primary pulley 3a reversely.
The starting apparatus 2 includes a lock-up clutch 2g for transmitting the engine power at the normal running directly to the output shaft 2f of the torque converter 2 without going through working fluid. The engagement and disengagement of the lock-up clutch 2g is controlled by a lock-up piston 2h.
In the torque converter case 2a, a so-called "apply chamber" 2j is formed on the left side of the lock-up piston 2h and a so-called "release chamber" 2k is formed on the right side thereof. When the working fluid supplied to the release chamber 2k flows into the apply chamber 2j, the lock-up piston 2h is moved to the left by the pressure difference between the release chamber 2k and the apply chamber 2j and the lock-up clutch 2g is disengaged to obtain the coupling condition of the torque converter 2. The working fluid flowing into the apply chamber 2j goes to the drain side after lubricating and cooling the torque converter 2b. Further, when the working fluid is supplied to the apply chamber 2j, since the working fluid is drained through the release chamber 2k, the pressure difference is generated between the apply chamber 2j and the release chamber 2k , thereby the lock-up piston 2h is moved to the right and the lockup clutch 2g is engaged to obtain the lockup condition.
The engagement and disengagement of the lock-up clutch 2g must be performed swiftly under any running conditions. Further, in general, the lock-up clutch 2g is engaged preferably at low speeds of the engine in order to enhance the power transmission efficiency. However, when the lock-up clutch 2g is engaged at the low speed area of the engine, the fluctuation of the engine speed is likely to increase especially in case of a small speed reduction ratio or the power train and the vehicle body may cause severe vibrations and noises due to a resonance at a particular rotational speed range.
Generally, the power transmission from the lock-up clutch 2g to the output shaft 2f of the torque converter is performed by way of a damper unit 21. In order to absorb fluctuations of the engine rotational speed effectively, it is necessary to increase the damper unit in size. The increase of size incurs not only an increase of manufacturing cost but also an up-sizing of the starting apparatus 2 due to an enlarged damper unit.
There is a known technique in which when the engine torque is short, a part of the driving force is transferred to the torque converter 2b by causing slips in the lockup-clutch 2g to reduce vibrations and noises.
Since the lock-up clutch 2g is operated by the relatively large amount of working fluid which flows between the apply chamber 2j and the release chamber 2k, further since this flow of the working fluid is controlled by changing over hydraulic circuits provided in the apply chamber 2j and the release chamber 2k, respectively, it is difficult to make a fine control with respect to the changeover timing of these hydraulic circuits and the pressure of the working fluid. Therefore, as far as the lock-up clutch 2g is controlled by the hydraulic circuits shared with other hydraulic circuits of the transmission, it is difficult to achieve a stable and accurate slip control in engaging or disengaging the lock-up clutch.
Accordingly, in order to obtain a lock-up control apparatus having a quick response and an accurate operation, it is desirable to incorporate a dedicated independent hydraulic circuit for the slip control. However, generally, providing a dedicated hydraulic circuit is impracticable due to not only the complication of hydraulic circuits but also an increase of the manufacturing cost.
Therefore, in order to achieve a practicable independent hydraulic circuit, the construction of the dedicated hydraulic circuit should be as simple as possible.
FIG. 3 is an example of the case where the engine 10 and the transmission 1 are arranged transversely, in which the overall width Ws of the engine 10 and the transmission 1 must be a size to be able to be accommodated in an engine room 6a.
Recent motor vehicles are equipped with a frame 7 in the engine room 6a for absorbing an impact on both sides of the vehicle in case the vehicle has a collision from the lateral direction. Further, a front tire 8 is arranged outside of the frame 7.
The frame 7 must have a sectional area sufficient to provide rigidity for safety and further the front tire 8 is required to move within as large steering angle as possible in order to obtain a small turning circle radius.
As a result of this, a space for accommodating the engine 10 and the transmission 1 in the engine room 6a tends to become small, therefore the overall width Ws including the engine 10 and the transmission 1 must be shortened as far as possible.
However, in the continuously variable automatic transmission 1 of the prior art, since the starting apparatus 2 and the forward and reverse changeover apparatus 4 are arranged between the engine 10 and the continuously variable transmission 3 according to the order of the transmission of power, it is technically difficult to shorten the width Wt of the continuously variable automatic transmission 1. Several means such as making a pulley ratio of the continuously variable transmission small, reducing the width of a belt wound around both pulleys, making walls thin, making clearances between components small and the like can be considered. However, any of these means may adversely affect the function of the continuously variable automatic transmission 1 or reduce rigidity and strength of the transmission.
As a result of this, it is difficult to incorporate these techniques for shortening the width of the automatic transmission 1 and further one or more other techniques for enhancing the responsibility and control performance of the lock-up control into the automatic transmission.